1. Field of the Invention
The present invention relates to a pressure control system for use, for example in controlling fluid pressure in the pressure pads of an alternating pressure mattress of a medical bed.
2. Discussion of Related Art
A known pressure control system for an alternating pressure mattress is shown in FIG. 1. The mattress is part of a medical bed and includes two series of inflatable cells which are interleaved, one series within the other. The cells are alternately inflatable to support a patient at different locations to prevent the formation of decubitus ulcers, known as bed sores. Typically, inflation and deflation cycles may last from under two minutes to over twenty minutes.
The pressure control system includes a compressor 10 for producing pressurised fluid, typically air, which is coupled to a rotor valve 12 via a conduit 14. The rotor valve 12 couples the air either to one of or to both of first and second supply conduits 16, 18. Each supply conduit 16, 18 is coupled to a respective series of inflatable cells 20, 22. These cells 20, 22 are interleaved such that a cell from one series is located between two cells of the other series.
Extending from the conduit 14 is a discharge conduit 24 which can be closed by a pivotable valve plate 26. This valve plate 26 acts against a load, in the form of a user adjustable coil spring 28. A pair of bellows 30, 32 is located to act against the valve plate 26. The first bellows 30 is coupled by tubing to the first supply conduit 16, while the second bellows 32 is coupled by tubing to the second supply conduit 18. Thus, the amount of air in each bellows 30, 32, and thus the shape thereof, is dependent upon the air pressure in the first and second supply conduits 16, 18.
Located at the opposite side of the bellows 30, 32 is a support plate 34 which is able to move in a direction towards the bellows 30, 32 when the pressure exerted by microswitch 36 exceeds the counter pressure produced by the bellows 30, 32. Thus, the movement of the support plate 34 and microswitch 36 act as a low pressure warning, for which a low pressure indicating lamp 38 is provided, coupled to the low pressure switch.
The bellows 30, 32 act together to control the overall pressure in the cells 20, 22. More specifically, as long as the combined fluid pressure in the cells 20, 22 is less than a predetermined threshold, the equivalent pressure in the bellows 30, 32 is insufficient to overcome the resisting force of coil spring 28. However, when the combined pressure is above the threshold, the bellows 30, 32 move the valve plate 26 off the aperture of the discharge conduit 24, thereby allowing discharge of fluid and a reduction in the mattress pressure until the valve plate 26 moves back into abutment with the discharge conduit aperture.
This pressure control system has been successful in use. However, the double bellows 30, 32 can sometimes fail to operate satisfactorily and the tubing provided in the system can lead to failure due to fluid leaks which may be inherent or caused by damage in use.